IMHO downwind does not equate to no wind departues. My reasoning:
No wind invariably involves transition to/from a hover in ground effect with a transition at some stage to/from translational lift, therefore both effects reducing power requirements.
Downwind departure or arrival may mean the ground effect in the hover is reduced (blown away) therefore a larger power requirement. In addition as you transition you will go from negative airspeed to positive airspeed passing through a zero airspeed point. At this point you will have zero airspeed and a groundspeed of variable speeds upto 30kts therefore achieving no ground effect. Although you may be close to the ground you will recieve no help from either translationl lift of ground effect therefore effectively requiring out of ground effect power requirements.
Into wind operations provide translational lift earlier, sometimes in the hover and this exceeds that lift achieved from ground effect, which may be blown behind you, therefore further reducing power required.
Summary:
You need more power to operate down wind, you need to understand the performance of your own machine, my experience lies just with three. This is a cracking thought provoking thread, may there be more like them!